On a more practical note,
here's my list of things to always take along when traveling. Note that
I'm talking about longer-term travel on a strict budget; if you're
only going for a few days, or you've got money to spare (or a corporate
sponsor), you may well want to take along less and, if necessary, buy
things wherever you're going.

Thank you to everyone who's made suggestions - if you think of anything else, please e-mail me!

There are several sections here: Everywhere Needs, Hostel Specifics, Practical Things (weather-based), and Optional (things which aren't necessary especially if you're a light packer, but which I personally would bring along).

And also, I assume you don't need my help to remember your toiletries,
underwear, and other such obvious things...

Everywhere
Needs

Everywhere.
No matter where you go, you will need this.

Swiss
Army Knife
(or imitation, of course)

Seriously.
Uncorking a wine bottle, adding an extra hole in your belt, getting
a splinter out of your finger, this is the tool of choice. Mock if
you like, but mine's been useful more times than I can count, and
so it tops the list. (If you're flying, make sure to put it in your checked luggage, or you risk losing it to airport security!)

Earplugs

You never
know. Snoring people in your dorm room, trains rattling by at 3 am,
the folks next door having loud sex - ignore it all comfortably.

Eye cover

If you're
not light-sensitive, ignore this one. I wake up as soon as it's light,
so I always bring an eye mask for too-light curtains or neon lights
next door.

Small
bottles for everything

Refill your
shampoo and stuff into small bottles. Saves space.

Handwash
Detergent

If I'm going
anywhere for a longer period, having a way to wash small things without
recourse to a laundromat is fabulous. In a small bottle of course.

Travel
Clothesline

If
you're washing more than the bare minimum at a time, you'll run out
of places to hang clothes. An elastic travel
clothesline like the one here or here can be invaluable.

Twist-ties, safety pins, and/or string

You never
know when you'll need them, but there's always sometime. Closing the
bag of sugar, keeping that rip in your pants secure, or tying the tattered ends of your zipper so your bag
can still close. MacGyver would never leave his house without twist-ties.
None of these things take up much room, making their space-to-usefulness ratio excellent.

Ziploc
Bags

For the wet
towel, for any non-dry substance, for food storage - always incredibly
handy.

Medications and other healthcare info

Seems obvious,
but don't forget whatever medications you might be on - especially
the ones you don't use all the time but really need sometimes... like
the vacation where I forgot my asthma inhaler. If you have it, information about blood type, vaccinations, etc. can be very useful - you'll be better off if you don't need it, but if you do need it you will hate not having it.

Moleskin

Unless you
have supershoes and you are a superwalker, you will likely get blisters.
And if you must have blisters, you must also have moleskin.

Q-Tips

Use 'em to clean your ears, or to clean out just about
anything.

Note Book

For everything: For
keeping a journal, for taking down the address of the woman whose
picture you just took so you can send her the picture later, for
writing down tips from other travelers, or just recording your impressions.
Also, I recommend using waterproof pens...

As many readers have pointed out, these are often harder to find than they should be (especially with language barriers), so it's a good idea to bring at least a month's supply. Even if you don't need them, someone else will, and having them will get you lots of good travel karma.

Sex Stuff

Might need
condoms? Bring 'em. No doubt there will be some available there, but
why waste the time figuring out where and how? And if you want anything
less mainstream than condoms - dental dams, lube, nipple clamps, whatever
- you're best off bringing it along. (And putting it in checked luggage,
probably.)

Sewing
Kit

Sure, it'll fix whatever
rips you put in the clothes. But when sterilized with alcohol swabs
or fire, those needles'll also help pop your blisters. And it takes
up just about no space at all. And you never know when string will
be handy.

Tissue/T.P.

Not just for runny noses, but for when your bus stops in a place that has no toilet paper.

Wet Wipes, Hand Sanitizer

Antibacterial wet wipes are something I always bring - the advantage of wipes over gel is that you can also clean off your camera, the local pay phone, or whatever else if need be. But if you're going to be on a plane and make sure you don't get everyone else's cold, bring something to wipe your hands with. (Yes, yes, I know Americans are paranoid about germs, but this really does help!)

Hostel
specifics

Ah,
shared showers, dorm rooms, bunk beds...

Towel

Any
towel will do, but a quick-drying
travel towel can save you from having to pack a wet towel - great
if you're on the move a lot, and saves space too.

Shower
shoes

Personally
I'd say it's must if you're sharing showers. Besides, slip-ons are
generally good, for those late-night walks to the bathroom and such.

Earplugs
& eye cover

If you're
sleeping in a dorm, take precautions.

Sheets/sleeping
bag

Not as essential
now that most places rent sheets. Yours will probably be cleaner than
theirs, though.

Salt &
pepper

If you're
self-catering, boy, it sucks when you can't get salt. Bring it or
put it first on your shopping list once you're there. And if you plan
to do any more serious cooking, bring small bags or baby food jars
of other spices - saves money, and is an easy way to jazz up your
usual boring self-catering fare.

Practical
things
in all weathers

...but
not all places have all weathers, so pick and choose!

Raincoat, poncho,
or umbrella

Because if
you pack it, you'll get good weather! (And you can often get super-small rain ponchos for cheap.) If you plan to hike, get a poncho that can cover your backpack too; that saved my butt on the Inca Trail.

Long underwear

Goofy, but
small, handy to pack, and keeps you deep down warm.

Windbreaker,
waterproof jacket

Not a bad
thing to have as a raincoat substitute if you won't face serious downpour.

Bathing
suit

You never
know when you'll need one. I needed one in winter in New Zealand.

Sturdy
boots

Waterproof,
in case you go tromping through mud or snow or puddles.

Sunblock, Sunglasses, Sun Hat

If you burn like I do, do not forget sun block; sun hat and sunglasses are also pretty important if you're going to be outside somewhere other than England.

One
nice outfit

You
never know what kind of random invitation you might get, but if you
pack something that can moonlight as casual yet vaguely elegant, you'll
be well prepared. For women, you may just need nice accessories and
some lip gloss.

Optional,
depending

What's
important to you? Who are you on your trip?

Photos

People you
meet want to see your family, friends, house, pets, who- and whatever
you care about. Bring them - especially if your language skills are
rusty, they help.

Camera

If it's digital,
bring a way to download pictures or extra disks so you can take more
pictures if you need to.

Cables, Cords, Etc.

Don't forget any cables you need to connect or power cameras, computers, or cell phones.

Playing Cards

A reader pointed out I'd left this off the list, and it's a serious omission. That 14-hour train ride would've gone a lot more slowly without playing cards, and they can be a great way to work around a language barrier.

Address
List

I bring a
list of everyone I'm going to send a postcard to, complete with their
addresses. I also bring along addresses of anyone I'm meeting on my
trip. One reader suggested bringing them along as pre-printed labels, which seems very handy.

Extra
film

Be careful
about how much this goes through x-ray machines; their effects are
cumulative, so if you put undeveloped 400-speed film through four
x-ray checks, you may see some damage. Might be worth either buying
film there or developing your film at your destination.

Cheap
(but not bad) perfume

Sort of like
the above Baby Powder tip, this one is to minimize your clothes washing.
Hang the bar-smoke-covered jeans in your bathroom. Spritz 'em with
perfume and let them hang an evening. And then? Voila. Quite a good
stopgap solution. I like Gap's "Om" flavor myself.

Gifts and Thank-You Cards

I usually take along
one or two gifts that are not specifically geared toward any one
person, assuming I will run into someone who will be so nice that
I'll want to give them this gift - a book, or something produced
where I live. Another reader suggested thank-you cards as a good way to show your appreciation. Obviously, when I know in advance
I'll be staying with someone or otherwise dependent on their kindness,
I also try to bring a specific gift for that person.

Salonpas

Love these.
They're little Japanese anti-ache patches, but I've discovered another
use for them. Caught in a smelly bus or room? Smell these instead.
Put 'em by your pillow or on your hand where you rest your head. Helps
in the archives, I'll tell you. Some folks recommend Vicks Vaporub
for the same reason.

Phone Card

Check out the phone situation before you go and decide what works best for you. A lot of companies offer phone plans that you can use by dialing a local number on a pay phone - just make sure you have the local number available! If you think you're using your cell phone, definitely do your research first, as a lot of phones aren't internationally compatible (my parents, who travel a lot, have at least three different phones and plans, one each for Europe, North America, and Asia); even if they are compatible, you may be financially better off getting a pre-paid local cell phone.

Guidebook

Not necessary,
but quite helpful.For South America, I recommend the South
American Handbook - no reason to argue with years of quality.
You could supplement them with cultural information from the Lonely
Planet or Rough Guide, though the LP is more often
out of date than accurate there. Let's Go is more appalling
there than anywhere. For New Zealand, either the Rough
Guide or Lonely Planet; see my New
Zealand travel tips for more details. For Europe, really, any one will do,
but please, don't take that bright Let's Go out of your pocket
in the middle of Paris, because we'll all cringe. Can we say it all
together now? "Don't go Let's Go!!" For elsewhere, I haven't really got any
advice! But hey, try the Lonely
Planet Thorn Tree for these and other questions!

This is
clearly a work in progress. I'm still thinking of and remembering
things. If you think of any that should be included, please e-mail
me and let me know!